In recent years, spectrophotometers have been actively used in a variety of fields. A spectrophotometer is a device that irradiates an object to be measured with light of different wavelengths, and measures the spectral distribution (spectral density) of the object being measured.
Generally, a spectrophotometer is provided with an optical unit and a control unit. The optical unit is mainly constituted by a spectrograph and a photodetector. The spectrophotometer may also be provided with an optical unit for use as a light source in addition to an optical unit for receiving light.
In the optical unit, the spectrograph separates transmitted light or reflected light from an object into light of specific wavelengths. The photodetector detects the separated light, and outputs a signal according to the amount of detected light. At this time, output is performed for each wavelength of detected light. In the control unit, the luminance of the detected light is then computed for each output signal. A spectral distribution is acquired as a result.
Incidentally, the use of a prism as the spectrograph constituting the optical unit is heretofore known. When a prism is employed as the spectrograph, however, the size of the optical unit and, by extension, the size of the spectrophotometer tends to increase, making it difficult to reduce the cost of spectrophotometers. In order to solve such problems, a spectrophotometer in which bandpass filters that only allow light of specific wavelengths to pass are used as the spectrograph has been proposed (see Patent Literature 1).
Patent Literature 1 discloses an optical unit that is provided with three sheet-like bandpass filters having different transmission wavelengths, and three light receiving elements corresponding to the bandpass filters. With this optical unit, the band bandpass filters are sheet-like in form, and are disposed such that their main surfaces coincide. The light receiving elements are each disposed so as to receive light transmitted through the corresponding bandpass filter. Therefore, the corresponding light receiving element outputs a signal according to the wavelength of received light.
With the optical unit disclosed in Patent Literature 1, the three sheet-like bandpass filters thus function as a spectrograph. Accordingly, use of the optical unit disclosed in Patent Literature 1 enables three types of light having different wavelengths to be measured with a simple configuration. A similar optical unit to the optical unit disclosed in Patent Literature 1 is also disclosed in Patent Literature 2.